This invention relates to a process for the fabrication of a coherently bonded mass of sand or like refractory grains or particles, for use as a disintegratable core for combined use with a complementary mold in foundry casting applications, with the core being used for forming a passage or opening in castings. The invention is directed more specifically to such a process employing microwave energy for curing the bonded and contoured mass of refractory particles.
Sand molds and cores for metal casting applications must meet two contradictory requirements: They must be of high strength before and during the molding of molten metal but should readily disintegrate and allow easy shakeout following the solidification of the metal. A variety of mold and core compositions, as well as methods of making molds or cores, have been suggested in an attempt to fulfill the above and other requirements.
A typical conventional method dictates the bonding of refractory particles such as silica sand with an organic binder such as phenolic resin, furan resin, urethane resin and urea resin, and thermally curing the bonded refractory particles. Shell molding is one well known curing method, wherein a mixture of sand and thermosetting phenolic resin is poured over a heated metal pattern, with the result that a thin shell of the mixture sticks to the hot pattern surface. Microwave heating is another curing method, as disclosed for example in Cole U.S. Pat. No. 4,331,197. The use of organic binders for holding refractory particles together is objectionable, however, as the molds made therefrom, whether they have been cured by the hot pattern or microwave method, do not necessarily contract upon solidification of the cast metal to an extent sufficient to prevent the hot cracking of the casting. Moreover, being synthesized from petroleum, the above numerated organic binders are generally expensive and subject to excessive cost fluctuations.
In view of the limitations and restrictions of organic binders, recent research and development efforts in the molding industry have again been centered on inorganic binders notably including water glass (sodium silicate). However, molds or cores as so far fabricated with use of inorganic binders have had a problem with regard to their disintegration after the setting of the cast metal. The quality of the castings made with such molds or cores has also been inferior in some instances to that of the castings fabricated with the molds or cores prepared with use of organic binders.
Take, for instance, the conventional carbon dioxide method wherein a body of sand bonded with water glass is hardened by a carbon dioxide gas. As much as four to six percent water glass has normally been added to the sand to assure mold strength. The high proportion of water glass has sometimes resulted in the sintering of the molds or cores at the time of casting. The sintered molds or cores do not, of course, readily disintegrate, do make shakeout difficult, and may partly stick fast to the castings. Further the molds or cores prepared as above have been relatively poor in surface stability. Still further, in reconditioning, for reuse, the sand reclaimed from the used molds or cores, there has been the danger of bringing about pollution or contamination problems.